1. Field of the Invention
The present invention is directed to a method for electrically contacting thin-film solar modules, and in particular to such a method for electrically contacting thin-film solar modules comprised of a plurality of side-by-side, strip-shaped solar cells.
2. Description of the Prior Art
Thin-film solar modules are usually produced directly on large-area substrates and are structurally integrated. Such modules have two electrode layers, and at least the electrode layer of the thin-film solar module facing toward the light incidence side is usually composed of a thin conductive oxide, particularly when the light must enter through a transparent substrate and a transparent, first electrode layer. Zinc oxide, stannous oxide or fluorine-doped indium tin oxide, for example, can serve as material for such transparent electrode layers. By structuring the thin-film solar modules, strip-shaped individual solar cells are produced which are connected in series to one another. The current which is generated by the incident light is then tapped at the two outermost cells.
Conductive oxides such as, for example, zinc oxide have a relatively small electrical conductivity. In order to maintain the electrical losses due to the increased series resistance low, the current path must be kept as short as possible in the cells. As a consequence, designers of such modules strive for an optimally low structural width in the strip-shaped structuring. The current tap at the two outer strip cells ensues by means of continuous contacting along the entire length of the outer strip cells to the extent possible.
In such known modules composed of amorphous silicon with zinc oxide electrodes, the electrical contacts at the outermost cells were soldered on. A solderable substrate, however, is required for this purpose. To that end, a current busbar is applied on the module substrate before the deposition of electrode layers and the active semiconductor layer. This current busbar permits soldering of a continuous contact strip over the entire cell length of the outermost cells, and also has a higher conductivity than the transparent, conductive oxide employed. The resistance loss is thus kept low even when the contact strip is only spot soldered.
The application of such current busbars usually ensues in a silkscreening process with heatable, conductive silver paste. Certain disadvantages, however, are associate with this known module manufacturing technique. During the heating process of the conductive silver paste, the wafers may be heated beyond the transformation point of the glass, and become pre-stressed due to rapid cooling. This limits this process to one employing pre-stressed wafers. In some circumstances, however, it may be more desirable to employ wafers which are not pre-stressed, in which case this known technique cannot be used. Although it is possible to avoid the pre-stressing by slowly cooling the glass, this is uneconomical because of the longer time required. The printing of the current busbars must ensue on a module edge length of more than one meter with a small tolerance with regard to the parallel alignment between the opposite busbars and relative to the substrate edges, so that the structuring of the module which is undertaken later with, for example, a laser, is not impaired. Since printing of the current busbars ensues at a small distance from the module or from the substrate edge, and since the printing is implemented under pressure, the screen which is employed for the printing is subject to increased wear. Techniques involving printing of a current busbar, therefore, are complicated and uneconomical.